Cultures of peripheral blood leukocytes from a series of human donors were stimulated in vitro against TNP-modified autologous leukocytes. Proliferative and cytotoxic T cells were obtained after primary, secondary and tertiary in vitro stimulation. The cytotoxic affectors exhibited TNP-dependent activity against modified murine lymphocytes. Blocking studies demonstrated inhibition of cytotoxic activity by addition of TNP-modified autologous targets, but not by TNP-lysine or TNP-conjugated chicken red blood cells. This establishes that the T-cells do not recognize TNP exclusively but TNP in association with human-specific self surface determinants. These determinants have been classified into three groups based on studies of direct lysis and cold target cell inhibition using panels of effectors from family members and selected unrelated donors. One class of determinants was polymorphic and associated with serologically defined HLA-A and -B locus antigens. A second class of determinants was not strongly associated with these antigens but was polymorphic and HLA-linked. A third class of "public" determinants existed for which there was little if any polymorphism among the donors studied. These results from the study of human TNP-dependent cell mediated cytotoxicity suggest that T cells recognize a variety of self-determinants in association with TNP.